Eleven-year old Mísa has the temperament of a proper boy and endless problems with the grown-ups. It is no wonder that he longs to become an adult without further delay. Mísa keeps house together with his father and has various duties. After a failed attempt at dish-washing he goes off to the research institute where his father is working on an important invention. He is trying to produce a substance that would make poultry and other animals bigger and thus help the struggle against famine in the world, but so far he has only managed to produce a shrinking substance. After his father leaves the laboratory, Mísa mixes up different ingredients and makes the growth substance. He drinks it and instantly becomes an adult. His father, conversely, turns into a toddler after drinking the shrinking substance with his beer by mistake.
Unfortunately the movie Just Because You're Grown-Up is not yet available on HBO Max.
| Writing | Sheila Och | Story |
| Art | Josef Šťovíček | Set Decoration |
| Art | Zdeněk Rozkopal | Production Design |
| Costume & Make-Up | František Čížek | Makeup & Hair |
| Writing | Pavel Vrba | Lyricist |
| Sound | Radomír Koutek | Sound |
| Directing | Zdeněk Zydroň | First Assistant Director |
| Writing | Milan Šimek | Screenplay |
| Crew | Josef Strnad | Dramaturgy |
| Camera | Emil Bártek | Camera Operator |
| Sound | Bohumír Brunclík | Sound Effects |
| Camera | Jiří Kolín | Director of Photography |
| Production | Vojtěch Kunčík | Unit Production Manager |
| Costume & Make-Up | Naďa Škapová | Costume Design |
| Editing | Antonín Štrojsa | Editor |
| Sound | Harry Macourek | Original Music Composer |
| Writing | Radim Cvrček | Screenplay |
| Art | František Santarius | Set Decoration |
| Production | Dušan Schaffer | Production Manager |
| Production | Věra Hábová | Assistant Production Manager |
| Costume & Make-Up | Karel Konrád | Makeup & Hair |
| Directing | Radim Cvrček | Director |